Strategy for Managing Horn Flies

Strategy for Managing Horn Flies

Dr. Megan Van Emon, MSU Extension Beef Cattle Specialist

As I’m sure many of you have noticed, it’s shaping up to be another bad fly year.  Horn flies are common on beef cattle here in Montana.  Annual control costs and cattle production losses can exceed $780 million annually in the U.S.

Horn flies spend the majority of their time on animals and will move from the back to the sides of the belly during the heat of the day.  Female horn flies deposit their eggs in fresh manure and can deposit up to 500 eggs during her life.  Horn fly larvae hatch and develop in manure.  Several generations of flies occur during the summer, with adult horn fly populations typically peaking during late summer.  When temperatures decline in the fall, horn fly pupae hibernate in the soil.

Horn flies prefer larger animals (cows, steers, heifers, and bulls), and tend not to bother calves until late summer.  One horn fly can bite 20 to 30 times per day, which can result in thousands of bites per day in large populations.  Cattle that are infested with over 200 flies will begin to bunch together in an attempt to escape the flies.  Large populations of horn flies cause changes in grazing behavior and can reduce feed intake, which ultimately decreases production.  Production losses include reduced feed intake, reduced feed efficiency, decreased milk production, and decreased weight gain.

Multiple methods are available for controlling horn fly populations.  Insecticides are the primary method for horn fly control as other methods are ineffective.  Methods available for cattle on pasture include dust bags, ear tags, sprays, pour-ons, boluses, and feed additives.

Dust bags, back rubbers, and oilers are most effective when cattle must pass under them on their way to water, feed, or mineral.  Dust bags can also be placed in loafing areas where they can be used free-choice.  Ear tags contain insecticides that allow for small amounts to be released over time by traveling through the hair coat when the animal is moving or grooming.  When using animal sprays to control horn flies, complete coverage and penetration to the skin is essential.  Sprays are easily applied, but require multiple treatments throughout the summer because the control of horn flies only lasts 3 to 4 weeks.  Pour-ons are also easily used and provide effective treatment for several weeks when used properly.  However, pour-on control may vary with weather and other factors.  Feed additives and boluses may be incorporated in mineral blocks, loose mineral or tubs.  The insecticides included as a feed additive pass through to the manure and kills the fly larvae.  Feed additives are most effective when consumed in sufficient amounts all season long.

For additional information and horn fly control contact your local Extension office to request a copy of the MontGuide by Greg Johnson, MSU Veterinary Entomologist.  The article is entitled “Horn Flies on Cattle: Biology and Management” (MT200912AG) and contains additional information about horn fly biology and available insecticide products.